Thursday, March 19, 2009

And a fair lay overview of ISRO's Chandrayaan

Today, India is one of the very few countries that have significant achievements to their credit in the arena of space. The Indian Space Research Organisation (ISRO) has designed, developed and built a variety of satellites. And, it has successfully launched many of them into their intended orbits. More importantly, the country has used its satellites for the rapid expansion of its national infrastructure including telecommunications, TV broadcasting, weather monitoring, education, public health, agriculture and rural development. More recently, India has provided many space-based services including launch services to foreign customers on a competitive basis. With ample experience and many successes in Earth orbit, ISRO took up Chandrayaan-1, its first bold step beyond Earth orbit into deep space.

Chandrayaan-1: The Goals

The primary objectives of Chandrayaan-1 are:1. To expand scientific knowledge about the moon2. To upgrade India's technological capability3. To provide challenging opportunities for planetary research to the younger generation of Indian scientists Chandrayaan-1 aims to achieve these well defined objectives through high resolution remote sensing of the moon in the visible, near infrared, microwave and X-ray regions of the electromagnetic spectrum. With this, preparation of a 3-dimensional atlas of the lunar surface and chemical mapping of entire lunar surface is envisaged.

Chandrayaan-1: The PayloadsChandrayaan-1 spacecraft carried 11 payloads (scientific instruments) to achieve its objectives. The instruments were carefully chosen on the basis of many scientific and technical considerations as well as their complementary/supplementary nature.

Of them, five instruments were entirely designed and developed in India, three instruments from European Space Agency (one of which was developed jointly with India and the other with Indian contribution), one from Bulgaria and two from the United States. Thus, Chandrayaan-1 is a classic example of international cooperation that has characterised the global space exploration programmes of the post cold war era.

The Indian payloads are:1. Terrain Mapping Camera (TMC): The aim of this instrument is to completely map the topography of the moon. The camera works in the visible region of the electromagnetic spectrum and captures black and white stereo images. It images a strip of lunar surface which is 20 km wide and resolution of this CCD camera is 5 m. Such high resolution imaging helps in better understanding of the lunar evolution process as well as in the detailed study of the regions of scientific interest. When used in conjunction with data from Lunar Laser Ranging Instrument (LLRI), it can help in better understanding of the lunar gravitational field as well. TMC was built by ISRO's Space Applications Centre (SAC) of Ahmedabad.

2. Hyperspectral Imager (HySI): This CCD camera is designed to obtain the spectroscopic data for mapping of minerals on the surface of the moon as well as for understanding the mineralogical composition of the moon's interior. Operating in the visible and near infrared region of the electromagnetic spectrum, it images a strip of lunar surface which is 20 km wide with a resolution of 80 m. The instrument splits the incident radiation into 64 contiguous bands of 15 nanometer (nm) width. HySI will help in improving the already available information on mineral composition of the lunar surface. HySI was also built by SAC.

3. Lunar Laser Ranging Instrument (LLRI): This instrument aims to provide necessary data for determining the accurate altitude of Chandrayaan-1 spacecraft above the lunar surface.It also helps in determining the global topographical field of the Moon as well as in generating an improved model for the lunar gravity field. Data from LLRI will enable understanding of the internal structure of the moon and the way large surface features of the moon have changed with time. The infrared laser source used for LLRI is Nd-YAG laser wherein Neodimium atoms are doped into a Yittrium Aluminium Garnet crystal. The wavelength of the light emitted by LLRI is 1064 nm. LLRI was built by ISRO's Laboratory for Electro Optic Systems (LEOS) of Bangalore.

4. High Energy X-ray Spectrometer (HEX): This is the first planetary experiment to carry out spectral studies at 'hard' X-ray energies using good energy resolution detectors. HEX is designed to help explore the possibility of identifying polar regions covered by thick water-ice deposits as well as in identifying regions of high Uranium and Thorium concentrations. Knowledge of the chemical composition of the various solar system objects such as planets, satellites and asteroids provides important clues towards understanding their origin and evolution. HEX uses Cadmium Zinc Telluride (CZT) detectors and is designed to detect hard X-rays in the energy range of 30 kilo electron Volts (keV) to about 270 keV. HEX was built jointly by Physical Research Laboratory (PRL) of Ahmedabad and ISRO Satellite Centre of Bangalore.

5. Moon Impact Probe (MIP): The primary objective of MIP was to demonstrate the technologies required for landing a probe at the desired location on the moon. Through this probe, it was also intended to qualify some of the technologies related to future soft landing missions. This apart, scientific exploration of the moon at close distance was also intended using MIP.

The 34 kg Moon Impact Probe consisted of a C-band Radar Altimeter for continuous measurement of altitude of the Probe above lunar surface and to qualify technologies for future landing missions, a Video Imaging System for acquiring images of the surface of moon from the descending probe and a Mass Spectrometer for measuring the constituents of extremely thin lunar atmosphere during its 25 minute descent to the lunar surface. MIP was developed by Vikram Sarabhai Space Centre of Thiruvananthapuram.

Chandrayaan-1 spacecraft carrying 11 scientific instruments weighed about 1380 kg at the time of its launch and is shaped like a cuboid with a solar panel projecting from one of its sides. The state of the art subsystems of the spacecraft, some of them miniaturised, facilitate the safe and efficient functioning of its array of scientific instruments.

The spacecraft structure was mainly built using composites and Aluminium honeycomb material. The Thermal subsystem consisting of paints, tapes, multi layer insulation blanket, optical solar reflectors, heat pipes, heaters and temperature controllers, ensures the proper functioning of the spacecraft by keeping its temperature within acceptable limits. The Mechanisms subsystem of Chandrayaan-1 spacecraft took care of the deployment of its solar panel and the steers of the dual gimballed antenna.

The spacecraft is powered by a single solar panel generating a maximum of 700 W. A 36 Ampere-Hour (Ah) Lithium ion battery supplies power when the solar panel is not illuminated by the sun. The Telemetry, Tracking and Command subsystem of Chandrayaan-1 working in S-band takes care of radioing the detailed spacecraft health information, facilitating the knowledge about spacecraft's position in space and allows the reception and execution of commands coming from Earth by the spacecraft.

Sun and star sensors as well as gyroscopes provide the orientation reference for spacecraft in space. The Attitude and Orbit Control subsystem, essentially the brain of Chandrayaan-1, consisting of a Bus Management Unit (BMU), reaction wheels and thrusters, ensures the proper orientation and stability of the spacecraft as well as in changing its orbit during different phases of its flight.

To make Chandrayaan-1 spacecraft to escape from orbiting Earth and to travel towards the moon, its liquid apogee motor (LAM) was used. Liquid propellants needed for LAM as well as thrusters were stored onboard the spacecraft.

Chandrayaan-1 spacecraft's Communications subsystem transmits the precious information gathered by its scientific instruments to Earth in 'X-band' through its Dual Gimballed Antenna.

The launch of Chandrayaan-1 took place at 6:22 am Indian Standard Time (00:52 UT) on October 22, 2008 from the Second Launch Pad at Satish Dhawan Space Centre, SHAR, Sriharikota in the Nellore district of Andhra Pradesh state. Sriharikota is situated at a distance of about 80 km to the North of Chennai.

Chandrayaan-1 spacecraft began its journey from Earth onboard India's Polar Satellite Launch Vehicle (PSLV-C11) and first reached a highly elliptical Initial Orbit (IO). In the Initial Orbit, the perigee (nearest point to Earth) was about 255 km and apogee (farthest point from the Earth) is about 22,860 km.

After circling the Earth in its Initial Orbit for a while, Chandrayaan-1 spacecraft was taken to five more elliptical orbits whose apogees were progressively higher a 37,900 km, 74,715 km, 164,600 km, 267,000 km and 380,000 km respectively. This was done by firing the spacecraft's Liquid Apogee Motor (LAM) at opportune moments when the spacecraft was near perigee. During this phase of the mission, the Terrain Mapping Camera (TMC), which is one of the eleven payloads of Chandrayaan-1 carried by spacecraft, was successfully switched ON and it took the pictures of the Earth and Moon. Additionally, Radiation Dose Monitor (RADOM), another payload of Chandrayaan-1, was also switched ON.

As it approached the apogee of its final Earth Bound Orbit at 380,000 km, the spacecraft passed at a distance of about 500 km from the Moon on November 8, 2008 since Moon had arrived there in its journey round the Earth.

At that time, the spacecraft's LAM was again fired. This slowed down the spacecraft sufficiently to enable the gravity of the moon to capture it into an elliptical orbit whose periselene (nearest point to the moon's surface) was at 504 km and whose aposelene (farthest point to the moon's surface) was at 7,502 km.

Following this, the height of the spacecraft's orbit around the moon was reduced in four steps. As a result of this, the periselene was reduced from 504 km to 200 km, and then to 182 km and finally to 100 km while the aposelene was reduced from 7,502 km to 255 km and then to 183 km and finally to 100 km. Thus, Chandrayaan-1 spacecraft reached its intended operational lunar polar orbit of about 100 km height from the moon's surface on November 12, 2008. After this, TMC sent excellent images of the lunar surface.

On November 14, 2008, the Moon Impact Probe (MIP), carrying the painting of Indian tricolor on its sides, was separated from the spacecraft and after a 25 minute journey, impacted the lunar surface near the South polar region of the moon at around20:31 Indian Standard Time (15:01 UT). Following this, the switching ON of the remaining nine payloads began. By mid December 2008, all the payloads had been switched on and tested.

By mid 2008, PSLV had repeatedly proved its reliability and versatility by launching 29 satellites into a variety of orbits. Of these, ten remote sensing satellites of India, an Indian satellite for amateur radio communications, a recoverable Space Capsule (SRE-1) and fourteen satellites from abroad were put into polar Sun Synchronous Orbits (SSO) of 550-820 km heights. Besides, PSLV has launched two satellites from abroad into Low Earth Orbits of low or medium inclinations. This apart, PSLV has launched KALPANA-1, a weather satellite of India, into Geosynchronous Transfer Orbit (GTO).

PSLV was initially designed by ISRO to place 1,000 kg class Indian Remote Sensing (IRS) satellites into 900 km polar SunSynchronous Orbits. Since the first successful flight in October 1994, the capability of PSLV was successively enhanced from 850 kg to 1,600 kg. In its ninth flight on May 5, 2005 from the Second Launch Pad (SLP), PSLV launched ISRO's remote sensing satellite,1,560 kg CARTOSAT-1 and the 42 kg Amateur Radio satellite, HAMSAT, into a 620 km polar Sun Synchronous Orbit. The improvement in the capability over successive flights has been achieved through several means. They include increased propellant loading in the stage motors, employing composite material for the satellite mounting structure and changing the sequence of firing of the strap-on motors.

PSLV-C11 is 44.4 metre tall and has four stages using solid and liquid propulsion systems alternately. The first stage, carrying 138 tonne of propellant, is one of the largest solid propellant boosters in the world. Six solid propellant strap-on motors (PSOM-XL), each carrying twelve tonne of solid propellant, are strapped on to the first stage. The second stage carries 41.5 tonne of liquid propellant. The third stage uses 7.6 tonne of solid propellant and the fourth has a twin engine configuration with 2.5 tonne of liquid propellant.

The 3.2 metre diameter metallic bulbous payload fairing protects the satellites and it is discarded after the vehicle has cleared dense atmosphere. PSLV employs a large number of auxiliary systems for stage separation, payload fairing separation and so on. It has sophisticated systems to control the vehicle and guide it through the predetermined trajectory. The vehicle performance is monitored through telemetry and tracking. The main modification in PSLV-C11 compared to its standard configuration is the use of larger strap-on motors (PSOM-XL) containing more propellants.

Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, designed and developed PSLV-C11. ISRO Inertial Systems Unit (IISU) at Thiruvananthapuram developed the inertial systems for the vehicle. Liquid Propulsion Systems Centre (LPSC), also at Thiruvananthapuram, developed the liquid propulsion stages for the second and fourth stages of PSLV-C11 as well as reaction control systems. SDSC SHAR processed the solid motors and carries out launch operations. ISRO Telemetry, Tracking and Command Network (ISTRAC) provides telemetry, tracking and command support during PSLV-C11's flight.